At the bottom of the barrel is "The results are in," with a 7.25 percent open rate.

"The shorter the subject line, the higher [the] chance of your email being opened," according to the infographic. "Remember: The more basic subject lines often perform best."

Consumers may be opening these types of emails the most because of how few words show in email clients on smartphones, says Scott Pielsticker, CEO and founder of ContactMonkey.

More advice about subject lines from ContactMonkey includes:

Use two words or fewer. "On average, these emails were opened more than twice and have an 80 percent open rate," according to Wednesday's announcement. Subject lines with three words or more have an average open rate of 15 percent.

Depending on the mobile screen, on which 40 percent of recipients open emails, recipients will see between four and seven words.